1. Field of the Invention
This invention relates to a tridentate ligand, more particularly to a 4,4′-dicarboxy-2,2′-bipyridine derived tridentate ligand. The invention also relates to a metal complex containing the tridentate ligand, and a dye-sensitized solar cell containing the metal complex.
2. Description of the Related Art
Photovoltaic cells, sometimes called solar cells, are being increasingly developed in the art due to the fact that sunlight to be converted into electrical energy via the photovoltaic cells is inexhaustible. A dye-sensitized solar cell (DSSC) is one of the photovoltaic cells having most potential. The dye contained in the DSSC is used to absorb visible and near infrared light from the sun to excite electrons. The excited electrons are then injected into a conduction band of a semiconductor electrode so as to produce photocurrent. Therefore, the photovoltaic conversion efficiency of the DSSC is affected primarily by the performance of the dye.
Ruthenium complex containing a tridentate ligand is commonly used as a dye for the dye-sensitized solar cell because superior synthetic productivity and better photovoltaic conversion efficiency can be obtained thereby as compared to a complex containing a bidentate or tetradentate ligand.
US 2009/0107552A1 discloses metal complexes used as a dye for the dye-sensitized solar cell. One set of the metal complexes has a formula of MX3L, wherein M is a transition metal ion, each X is independently a monodentate ligand selected from the group consisting of thiocyanate, isothiocyanate, and optionally substitutable pyridine, and L is a tridentate ligand having a formula defined therein. A compound having a following formula is specifically illustrated:
The extinction coefficient of the compound is about 13,600 at λmax of 612 nm. The photovoltaic conversion efficiency of a solar cell containing the compound is about 3.9%.
As shown in the above formula, there is only one carboxyl group (COOH) or carboxylate ion (CO2−) in the terpyridine ligand of the ruthenium complex. Therefore, the terpyridine ligand can not be effectively bonded to the anode made of titanium dioxide nanoparticles. Furthermore, a conjugate group (i.e., a dithiophene group) intervening between the carboxyl group (or the carboxylate ion) and the terpyridine ligand is required, thereby resulting in a long distance between ruthenium and the carboxyl group (or the carboxylate ion) which is to be bonded to titanium dioxide. It might be due to such a relatively long distance which the generated photoelectrons have to travel, that a reduction in the voltage and the current intensity produced by the solar cell is possible. Furthermore, it is desirable to enhance the photovoltaic conversion efficiency and the absorption at both the visible and near infrared regions of the aforesaid ruthenium complex. Such a stringent demand for panchromatic absorption has thus been achieved by addition of thiocyanate ligand. Moreover, the thiocyanate ligands contained in the ruthenium complex have a relatively weak coordination bonding strength, and thus can not firmly chelate with the ruthenium atom. Therefore, the efficiency and the lifespan of the dye-sensitized solar cell produced thereby need further improvement.
On the other hand, an anionic complex {Bu4N}3[Ru(Htctpy)(NCS)3], H3tctpy=4,4′,4″-tricarboxy-2,2′:6,2″-terpyridine, known as a black dye or a N749 dye, has also been used in the art of the dye-sensitized solar cell. Three carboxy groups are contained in the anionic complex.
It is found by the inventors of the present invention that terpyridine ligand substituted with only two carboxy groups can vest the complex with desirably superior properties, e.g., bonding to the TiO2 electrode. In accordance with the present invention, it is uncovered that by reducing the number of carboxy as the substituent group on terpyridine from three to two, while incorporating a highly conjugated n-electron donating appendage at the third carboxy-free pyridine moiety, the solar energy harvesting capability can be significantly and desirably improved, without encountering the drawbacks in the prior art as mentioned above.